They filter out pretty much anything that has to do with sex, whether it's removal from Suggest or disabling instant search ("Press Enter to search").
It's hardly surprising that Google would remove it from these lists.
I could see porn sites not legitimately making that page. First of all the categories are nearly all "Fastest Rising", you can easily imagine porn queries being less volatile. Also, despite porn's popularity, there are basically no porn stars famous enough to pull the same number of queries as a Katy Perry or a Shakira.